The invention relates generally to an optical scanner and more specifically concerns an optical scanner used in a laser velocimeter (LV) system incorporating an afocal lens system in combination with a movable scanning lens.
A variety of anemometers are in use for measuring the magnitude of fluid flow velocity in a wind tunnel. Among these are pitot tubes, hot wires and reference beam laser velocimeters. The reference beam laser velocimeter measures the doppler frequency shift induced in scattered light from a moving target object, such as a dust particle or smoke particle suspended in a fluid stream. The magnitude of doppler shift is indicative of the velocity of the target object and the fluid. This device is especially useful where fine resolution is desired as in the measurement of velocity in fluid boundary layers adjacent to model surfaces in wind tunnel tests. In the LV system parallel laser beams are focused by lenses to a sharp image where they intersect. Back scattered light from the small volume around the image intersection point, of focus point, is collimated by reverse transmission through the lens system.
When it is desired to vary the spatial location of the focus point some sort of scanning system must be employed.
There are a number of optical scanning systems in the prior art for focusing parallel LV laser beams at points in space. These devices effect a scan by either moving the focusing element or by employing a focusing element of variable focal length. The present invention utilizes an afocal lens combination and a movable scanning lens. An afocal lens combination is a pair of lenses positioned apart a distance precisely equal to the sum of their focal lengths making the focal length of the combination infinite. Two important characteristics of an afocal lens combination are, firstly, that light entering the combination at a given angle with respect to the optical axis of the system will leave the system at a given angle which is dependent only on the magnification of the afocal combination angle and, secondly, that the magnification of the afocal combination is equal to the ratio of the focal lengths of the lenses and is independent of the object distance. These characteristics when applied to the intersecting laser beams in an LV system result in a constant crossbeam intersection angle and constant focal volume dimensions. Herein lies the advantage of the invention over the prior art scanners utilizing focusing elements which inherently cannot maintain constant crossbeam angle and focal volume. If as in these systems, the crossbeam angle and the focal volume are not maintained constant, the doppler shift in the scattered light is dependent on the focal volume location as well as the velocity of the scattering particle and require calibration of equipment for each focal position.
The present invention requires no calibration for each focal position during a scan. In the invention, the parallel beams pass through the scanning lens, through the afocal lens combination and are focused somewhere beyond on the optical axis. Moving the scanning lens a distance, d, with respect to the afocal combination moves the focal volume the distance d times the square of the magnification of the afocal lens combination along the optical axis.
Herein lies the advantage of the invention over the prior art scanners utilizing a movable focus element alone to effect a scan. Such a system moves the target volume a distance equal to the distance the focusing element is moved or it moves the target volume nonlinearly with the focusing element whereas the present invention effects a long scan of constant magnification from a relatively short movement of the scanning lens.
An object of the present invention is a method and apparatus for focusing light beams at points in space.
A further object of the present invention is a method and apparatus for focusing light beams at various points in space at constant focal volume and constant crossbeam angle.
A further object of the present invention is a method and apparatus for focusing light beams at various points whereby the scanning distance is longer than the scanning lens movement distance.
A further object of the present invention is an apparatus yielding the foregoing advantages through the combination of an afocal lens combination and a movable scanning lens.
Other objects and advantages of the present invention will be readily apparent from the following description and drawing which illustrate the preferred embodiment of the invention.